Differential effects of Bcl‐2 overexpression on fibre outgrowth and survival of embryonic dopaminergic neurons in intracerebral transplants

Abstract

The causes of death of transplanted neurons are not known in detail, but apoptotic mechanisms involving caspase activation are likely to play a role. We examined whether overexpression of the anti‐apoptotic protein Bcl‐2 may enhance the survival of dopaminergic [tyrosine hydroxylase (TH)‐immunoreactive] grafted neurons. For this purpose, we prepared cells from embryonic day 13 ventral mesencephalon (VM) of mice overexpressing human Bcl‐2, or from their wild‐type littermates. The bcl‐2 transgene was strongly expressed in these cells, and resulted in protection of neuronal cultures from death triggered by serum deprivation or exposure to staurosporine. To model pretransplantation stress more closely in vitro, we stored dissociated embryonic mesencephalic cells for 8 h in the same type of medium used for intracerebral transplantation. This resulted in massive cell death as quantified by lactate dehydrogenase (LDH) release, and increased DNA fragmentation. Although this cell loss was strongly reduced by a caspase inhibitor, Bcl‐2 had no significant protective effect. Finally, mesencephalic cell suspensions were xenografted into the striatum of immunosuppressed hemiparkinsonian rats. Neither the survival of TH‐immunopositive transplanted neurons nor the functional recovery of the rats was improved by Bcl‐2, although the Bcl‐2 protein was strongly expressed in transgenic grafts 5 weeks after implantation, and dopaminergic fibre outgrowth from the grafts was significantly improved. These data suggest that cell death in neuronal transplants involves apoptotic mechanisms that can bypass negative regulation by Bcl‐2.